Method for thinning wafers that have contact bumps

ABSTRACT

In accordance with the foregoing objects and advantages, the present invention provides a fabrication device that may be used during the grinding operation of the fabrication process. The fabrication device comprises a socket plate that includes a plurality of cavities formed therein that correspond in position and number to the solder (or other conductive material) bumps formed on the front surface of a product wafer.

BACKGROUND OF THE INVENTION

1. Field Of Invention

The present invention relates generally to semiconductor wafers, andmore particularly to the devices and methods for fabricating wafershaving solder bumps formed on one surface thereof.

2. Background

The desire for smaller, thinner, lighter electronic components hasdriven the semiconductor packaging industry to develop tools andprocesses for wafer backside thinning by grinding and polishing beforethe wafers are diced into chips. As packaging assemblers develop moresophisticated multi-chip packages containing multiple die which areoften stacked in a 3D assembly, the need for thinner die becomes moreacute.

Wirebond stacked die packages may contain die thinned to less than 200um, with even more aggressive thinning anticipated in the future.Because the surface of a wafer being processed for wire bondedinterconnects is planar, the mechanical process of backside grinding andsurface finishing is capable of removing greater than 90% of theoriginal wafer thickness without breakage or lattice damage. Thesetechniques have enabled the manufacture of extremely thin stackedwirebond package assemblies for advanced electronics.

With increasing semiconductor complexity comes the desire for greatersignal bandwidth, which requires that the semiconductor chips havehigher interconnect density from the die to the module substrate, or inthe case of stacked assemblies, from die to die. Higher performanceinterconnects are achieved by switching from wirebond to solder bumpconnections. However, since the solder bumping process must be done withunthinned wafers, the 30-100 um solder bump feature creates significantchallenges to obtain wafer backside thinning comparable to what isachieved today for wirebond wafers.

Thinning of wafers with solder bumps is done today by using a complianttape on the wafer front side to conform around, cushion, and protect thesolder bumps during the grinding process. This is reasonably effectivefor final wafer thicknesses greater than 300 um. For thinner dimensions,a tape with higher compliancy allows for replication of the underlyingbump features especially in dissimilar pattern density regions of thebumps. Conversely, a tape with lower compliancy may also reflect patterndensity differences as well as posing difficulty in removal. Theseissues have prevented the packaging industry from embracing thin singleand multi-chip stacked die packaging for solder bumped wafers.

3. Objects and Advantages

It is therefore a principal object and advantage of the presentinvention to provide a fabrication device and method for permitting theeffective thinning of bumped wafers through a grinding operation.

It is another object and advantage of the present invention to provide afabrication device that permits backside grinding of bumped wafers tothinner dimensions than is presently feasible.

It is a further object and advantage of the present invention to providea fabrication device that is easily adapted to existing fabricationprocesses.

Other objects and advantages of the present invention will in part beobvious, and in part appear hereinafter.

SUMMARY OF THE INVENTION

In accordance with the foregoing objects and advantages, the presentinvention provides a fabrication device that may be used during thegrinding operation of the fabrication process. The fabrication devicecomprises a socket plate that includes a plurality of cavities formedtherein that correspond in position and number to the solder (or otherconductive material) bumps formed on the front surface of a productwafer. Prior to the back-grinding process that thins the product waferbefore it is diced into chips, the front surface of the product wafer isplaced into engagement with the socket plate with each of the solderbumps being positioned in a respective socket. The dimensions of thesockets are slightly larger than the dimensions of the bumps in order toprovide a protective housing. When the bumps are positioned within thesockets, the front surface of the product wafer contacts the opposingsurface of the socket plate. The backside of the product wafer may thenbe ground without exerting any excessive force on the bumps. Inaddition, the socket plate maintains the product wafer in a stableposition during the grinding operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated byreading the following Detailed Description in conjunction with theaccompanying drawings, in which:

FIGS. 1 a and 1 b are front elevation, enlarged, cross-sectional viewsof the preferred and alternate embodiments of the present invention,respectively;

FIG. 2 is a perspective view of a typical mask used in connection withthe present invention; and

FIGS. 3A-3D are sequential, front elevation, cross-sectional views ofthe socket plate of the present invention in its various stages offabrication.

DETAILED DESCRIPTION

Referring now to the drawing figures in which like reference numeralrefer to like parts throughout, there is seen in FIG. 1 a fabricationsystem, designated generally by reference numeral 10, employed duringthe grinding operation of a wafer fabrication process in order to thin aproduct wafer 12 prior to its being diced into chips. Product wafer 12is of the type that includes a plurality of conductive bumps 14 formedin an array on its front surface 16 in a predetermined pattern, and anopposing back surface 18 that is adapted to be engaged by a grinding padduring the grinding operation of the fabrication process in order tothin the wafer to a predetermined thickness (generally of about 300 um).

Bumps 14 may be formed on front surface 16 by using a metal mask 20 thatincludes apertures 22 formed therethrough that are of about the samedimension and arranged in the same predetermined pattern as is desiredfor the bumps (the bumps are of a height, h, generally on the order of50 um-100 um, and a diameter, d, generally on the order of 60-120 um).

Fabrication system 10 further comprises a socket plate 24 that includesa front surface 26 through which (See FIG. 1 a illustrating holes formedentirely through plate 24) or in which (see FIG. 1 b illustrating blindholes formed in the front surface 26 of plate 24) a plurality ofcavities (or “sockets” and which may be either through holes (see FIG. 1a) or blind holes (see FIG. 1 b)) 28 are formed. Cavities 28 are ofheight, H, and diameter, D, that are slightly larger than the height, h,and diameter, d, of product wafer 12, and are equal in number andarranged in the same predetermined pattern as bumps 14, therebypermitting bumps 14 to be inserted into corresponding cavities 28 forpurposes described hereinafter (bumps 14 are generally in the range of60-120 um in diameter and 50-100 um high, and cavities 28 should beabout 10-30 um greater in those dimensions than bumps 14).

Socket plate 24 may be fabricated from a silicon wafer, but may also befabricated from a metal plate, such as one made from Molybdenum orstainless steel, or a plastic plate that may be etched. Preferably,plate 24 is also relatively thin (on the magnitude of about 0.1-1 mm inthickness), and is round with about the same diameter as bumped productwafer 12.

The following paragraph describes a method of fabricating a socket plateout of a silicon wafer utilizing the same Molybdenum mask used to formthe solder bumps (C4 balls) on the product silicon wafer. Cavities 28are formed by first coating a CVD (chemical vapor deposition) oxidelayer 30 with a positive resist material 32, such as TOK3227manufactured by Tokyo Ohka Kogyo Co., Ltd. (See FIG. 3A). Next, mask 20is flipped from its orientation used to create bumps on surface 16 andis placed over CVD oxide layer 30. The resist 32 is exposed through mask20 and then removed by the developer, thereby leaving openings 34 thatcorrespond to the pattern of bumps 14 (See FIG. 3B). CVD oxide layer 30is then etched using resist material 32 as a mask. The resist material32 may then be removed, leaving an oxide hard mask (see FIG. 3C). A deepsilicon etch may then be performed (if pate 24 is made of silicon) onthe socket plate 24 until the cavities 28 are formed to a depth H deepenough to receive bumps 14 therein (see FIG. 3D) (as stated earlier, thecavities 28 may be formed entirely through plate 24 or partially throughplate 24 with the important feature being that the depth H and diameterD are sufficient to accommodate bumps 14). If a material other thansilicon is used for plate 24, such as metal or plastic, cavities 28 maybe formed using well known, traditional etching techniques for metalplates, or by mechanical or laser drilling operations.

Prior to grinding the back surface 18 of product wafer 12, socket plate24 is placed on a vacuum chuck 36 and is put into registry with productwafer 12 with each bump 14 being positioned within a correspondingcavity 28, with front surface 16 of product wafer 12 contacting surface26 of socket plate 24. Back surface 18 may then be ground in the typicalmanner. The support provided to product wafer 12 by socket plate 24,permits the grinding force to be applied to back surface 18 evenly andprevents bumps 14 from incurring excessive forces from the grindingoperation. It should be noted, however, that with respect to thepreferred embodiment illustrated in FIG. 1 a, the vacuum created byvacuum chuck 36 is effective at maintaining bumps 14 in position withincavities 28 due to the free flow of air through the cavities 28, whereaswith the embodiment of FIG. 1 b bumps 14 are maintained in stableposition within cavities 28 due primarily to the force created by thegrinding device.

1. A fabrication device for use during the grinding operation of aproduct wafer having a first surface on which a plurality of conductivebumps are formed in a predetermined pattern and a second surface towhich a grinding force is to be applied, said fabrication devicecomprising: a. a socket plate having first and second surfaces; and b. aplurality of cavities formed in said first surface of said socket platethat are arranged in the predetermined pattern and correspond in numberwith the plurality of conductive bumps formed on said first surface ofsaid product plate.
 2. The fabrication device according to claim 1,wherein said plurality of cavities are defined by holes formed entirelythrough said socket plate.
 3. The fabrication device according to claim1, wherein said plurality of cavities are defined by blind holes formedin said first surface of said socket plate.
 4. The fabrication device ofclaim 1, wherein said socket plate is composed of a metal plate.
 5. Thefabrication device of claim 4, wherein said metal plate is composed ofMolybdenum.
 6. The fabrication device according to claim 1, wherein saidconductive bumps are of a predetermined height in the range of 50-100um, and of a predetermined diameter in the range of 60-120 um indiameter, and said plurality of cavities are of a height in the range of80-130 um, and of a predetermined diameter in the range of 90-150 um indiameter.
 7. A method for thinning a product wafer having a firstsurface on which a plurality of conductive bumps are formed in apredetermined pattern and a second surface to which a grinding force isto be applied to effect the thinning, said method comprising the stepsof: a. providing a socket plate having first and second surfaces and aplurality of cavities formed in said first surface thereof that arearranged in the predetermined pattern and correspond in number with theplurality of conductive bumps formed on said first surface of saidproduct wafer; b. placing said product wafer in registry with saidsocket plate by inserting said plurality of conductive bumps incorresponding ones of said plurality of cavities; and c. applying agrinding force to said second surface of said product wafer.
 8. Themethod for thinning a product wafer according to claim 7, furthercomprising the step of placing said socket plate on a vacuum chuck priorto placing said product wafer in registry with said socket plate.
 9. Themethod for thinning a product wafer according to claim 7, wherein saidplurality of cavities are defined by holes formed entirely through saidsocket plate.
 10. The method for thinning a product wafer according toclaim 7, wherein said plurality of cavities are defined by blind holesformed in said first surface of said socket plate.
 11. A system forperforming the grinding operation in a wafer fabrication process,comprising: a. a product wafer having first and second surfaces and aplurality of conductive bumps formed on said first surface and arrangedin a predetermined pattern; and b. a socket plate having first andsecond surfaces and a plurality of cavities formed in said first surfacethereof arranged in said predetermined pattern and corresponding innumber with the number of said plurality of conductive bumps.
 12. Thesystem for performing the grinding operation in a wafer fabricationprocess according to claim 11, wherein said plurality of cavities aredefined by holes formed entirely through said socket plate.
 13. Thefabrication device according to claim 1 1, wherein said plurality ofcavities are defined by blind holes formed in said first surface of saidsocket plate.
 14. The fabrication device of claim 11, wherein saidsocket plate is composed of a metal plate.
 15. The fabrication device ofclaim 14, wherein said metal plate is composed of Molybdenum.
 16. Thefabrication device according to claim 11, wherein said conductive bumpsare of a predetermined height in the range of 50-100 um, and of apredetermined diameter in the range of 60-120 um in diameter, and saidplurality of cavities are of a height in the range of 80-130 um, and ofa predetermined diameter in the range of 90-150 um in diameter.
 17. Amethod for forming a socket wafer that is used in a process for thinninga product wafer that includes a first surface on which a plurality ofconductive bumps are arranged in a predetermined pattern determined by afirst mask, comprising the steps of: a. providing a plate ofpredetermined material having first and second opposing surfaces; b.coating said first surface of said plate with a CVD (chemical vapordeposition) oxide layer with a positive resist material; c. flippingsaid first mask from its orientation used to create said conductivebumps and placing it on said first surface of said plate in coveringrelation to said resist material; d. removing said resist material thatis exposed through said first mask; e. removing said first mask fromsaid plate; f. etching said CVD oxide layer using said resist materialas a second mask to form a plurality of cavities in said plate; and g.removing the remainder of said resist material.
 18. The method accordingto claim 17, wherein said plurality of cavities are defined by holesformed entirely through said socket plate.
 19. The method according toclaim 17, wherein said plurality of cavities are defined by blind holesformed in said first surface of said plate.
 20. The method according toclaim 17, wherein said plate is composed of a predetermined metal. 21.The method according to claim 20, wherein said metal plate is composedof Molybdenum.
 22. The method according to claim 17, wherein saidconductive bumps are of a predetermined height in the range of 50-100um, and of a predetermined diameter in the range of 60-120 um indiameter, and said plurality of cavities are of a height in the range of80-130 um, and of a predetermined diameter in the range of 90-150 um indiameter.